Many areas of polymer processing require multiple layers of different polymers to be co-extruded into a single tubular form. One example is the blown film process which is used to make most of today's commodity bags and also high barrier food packaging. Although multi-layer packaging can be made from co-extruded flat film, using a tubular form presents fewer sealing operations, results in less trim scrap and is more conducive to certain product shapes.
Tubular forms are used in many applications including the production of multi-layer pipe or tubing, pipe coating, wire coating, and the production of multi-layer parisons for blow molding. Tubular parisons are used in making containers of various shapes as annular dies are typically easier to manufacture than dies of other shapes, such as oval or square. Annular co-extrusion dies are commonly used to process high volume commodity resins as well as relatively low volumes of barrier type resins.
Annular co-extrusion dies are generally of one of two arrangements; namely axially fed and radially fed. In either type of arrangement, melt is introduced into an inlet port from where it has to be evenly distributed about the circumference of an annular outlet. Good flow distribution is essential to forming film having layers which are uniform in thickness, appearance and structural integrity. In axially fed co-extrusion dies, melt is fed in a direction parallel to the axis of the tubular form to be extruded. Each layer is formed between respective die elements which are generally concentrically disposed in a manner analogous to cups of different diameter stacked one within an other. The individual layers are merged upstream in an extrusion passage through which the co-extruded film is discharged.
In radially fed co-extrusion dies, melt distribution blocks are stacked one behind another along a die axis and melt is fed radially relative to the die axis into a respective inlet port in each melt distribution block. The melt distribution blocks distribute the melt about a central mandrel and discharge the melt in an axial direction into an extrusion passage between the melt distribution blocks and the mandrel. Each consecutive melt distribution block applies an overlying melt layer to the melt moving along the extrusion passage.
Axially stacked radially fed co-extrusion dies are advantageous in that it is relatively simple to vary the number of layers by varying the number of “modules” stacked along the die. Furthermore, each level presents a similar area and the levels are more easily thermally isolated than possible with axially fed co-extrusion dies in which heat from one die element is difficult to isolate from adjacent die elements. Even melt distribution is however a much more challenging problem with radially fed co-extrusion dies because of a much shorter axial distance being available for melt equalization and the requirement to redirect melt flow from a radial to an axial direction after the melt has been distributed into a thin film.
It is an object of the present invention to provide a radially fed multilayer extrusion die which is effective in providing a uniformly thick film of melt to an extrusion passage.
It is a further object of the present invention to provide a melt distribution block for a radially fed multilayer extrusion die which can accept and combine two different types of melt.
It is yet a further object of the present invention to provide a melt distribution block for an extrusion die having a matched pair of distribution passages so configured and oriented as to cause an averaging of extruded film thickness by matching high flow areas of one of said pair of passages with lower flow areas of the other of said pair of passages.